Note that this skill also applies to missile launchers (rockets / missiles / cruise missiles / torpedoes) and not just guns.The cumulative effect of WU V and AWU to IV really makes a difference to fitting, especially when you start encountering the greater fitting demands of T2 equipment.

Nanite paste can be used to repair module damage from overheating on the fly, while in space. This skill speeds up those repairs. This is a nice skill to have if you're a PvP pilot, but it's not a high priority.

The skill required to use nanite paste to repair module damage from overheating on the fly. The reduction in nanite consumption will save you money in the long run. Good skill for PvP pilots, especially ones who go on longer roams and so rely on paste more.

The basic skill of this whole category, and a key fitting skill. Quickly trained. 25% more powergrid is a big boost to your ability to fit more, better equipment on your ship.

Resistance Phasing

Improves control over, and flow between, nano membranes that react to damage by shifting resistances.
Reduces cycle time of Reactive Armor Hardeners, Flex Armor Hardeners and Flex Shield Hardeners by 10% per level and capacitor need by 15% per level.

Thermodynamics

Advanced understanding of the laws of thermodynamics. Allows you to deliberately overheat a ship's modules in order to push them beyond their intended limit. Also gives you the ability to frown in annoyance whenever you hear someone mention a perpetual motion unit. Reduces heat damage by 5% per level.

The overheating of active modules can have a significant effect in combat situations, the effects of overheating modules range from an increase in the damage output of weapons, larger speed boosts from propulsion modifiers, an increase in the range of certain modules, better resists, an increased rate of repair and more. Higher thermodynamics skill allows you to overheat modules for longer before burning them out.

Weapon Upgrades

Knowledge of gunnery computer systems, including the use of weapon upgrade modules. 5% reduction per skill level in the CPU needs of weapon turrets, launchers and smartbombs.

Note that this skill also applies to missile launchers (rockets / missiles / cruise missiles / torpedoes) and smartbombs and not just guns. A very useful fitting skill which every combat pilot should train. Level V is no longer required for the Advanced Weapon Upgrades skill.